function plotShade(idx, dx, c, a)
%PLOTSHADE Summary of this function goes here
%   Detailed explanation goes here
    idx = reshape(idx, 2, 1);
    XX = [idx idx+1] .* dx;
    X = [XX(1,1) XX(1,2) XX(1,2) XX(1,1)];
    Y = [XX(2,1) XX(2,1) XX(2,2) XX(2,2)];
    fill(X, Y, c, 'FaceAlpha', a, 'EdgeColor', 'none');
end

